1. Field of the Invention
This invention relates generally to telecommunication systems, and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
Release 5 of the Universal Mobile Telecommunication System (UMTS) protocol permits each mobile unit, sometimes referred to as a user equipment or UE, to autonomously transmit messages to a base station (or Node-B) on a dedicated channel (or DCH). Most transmissions from the mobile unit are scheduled by the base station, which may result in scheduling gain. However, UE can autonomously transmit messages at any time without being scheduled by the base station. The autonomous transmission may cause interference to other channels associated with other mobile units, thereby increasing the rise-over-thermal at the base station, as well as having other undesirable effects that may offset a portion of the scheduling gain.
The autonomous transmissions are typically limited to certain rates, at least in part to limit the potential interference and control the rise-over-thermal. For example, each mobile unit may transmit in autonomous mode at a minimum transmission rate of at least 8 kbps. By limiting the data rate of the autonomous transmissions, the transmission power required of the mobile units may also be limited. Thus, the potential interference and rise-over-thermal may be kept with a desired range. However, the mobile unit may also transmit autonomously at higher bit rates with correspondingly higher channel powers, if it is determined that the potential interference and/or rise-over-thermal are not above some threshold level.
Future generations of mobile telecommunications standards may include an “enhanced” dedicated channel (EDCH). The enhanced dedicated channel may support one or more Transmission Time Intervals (TTIs), which may also be referred to as frame sizes. For example, UMTS release 6 may support both a 10 ms TTI and a 2 ms TTI, although the 2 ms TTI is not mandatory. The reduced frame sizes supported by future generations of mobile telecommunication standards may require a higher data transmission rate and, consequently, higher mobile unit transmission powers. For example, using typical assumed values for the size of a Radio Link Control (RLC) Packet Data Unit (PDU) and associated transmission overhead, the minimum data transfer rate necessary to transmit a data packet in a 2 ms TTI would be about 176 kbps. At this rate, the required channel power, or Ec, may be higher than could be supported by the mobile unit without causing unacceptably high levels of interference and/or rise-over-thermal at a receiving base station.
One proposal for reducing minimum data transfer rates includes restricting autonomous transmissions to a predetermined subset of TTIs. A subset of TTIs would then be assigned to each mobile unit when a communication link is established. However, this proposal has a number of drawbacks. For one example, scheduling gain may be reduced because the mobile units are forced to transmit during the predetermined TTIs, regardless of the condition or quality of service of the uplink channel during the TTI. Thus, the mobile unit may have to transmit autonomously while the uplink channel is temporarily degraded by a transient effect, such as fading, instead of rescheduling the autonomous transmission so that it is transmitted in a different TTI having a better channel condition.
The present invention is directed to addressing the effects of one or more of the problems set forth above.